Bone heat weighting problem, texture paint problem, blending model
This model is very messy. Firstly append it to a new file to fix some viewport messed up settings and make it an appropriate size.



Secondly, automatic weights freak out because of the inverted normal, recalculate them.



Thirdly, clean up weights and pose and reapply parenting.



The model is still a mess 



It is better to take the original and optimize it. https://free3d.com/3d-model/leprechaun-v1--543940.html

Make him T-pose for better results.



Optimize geometry. Better to remesh it. But I won't .



You can also unwrap and bake normals.





The automatic weights are not very suited for this. It might be better to use something more advanced, which is actually intended as an auto rigger. For example https://www.mixamo.com/



Choose T-pose to export



Or paid addon for Blender: https://blendermarket.com/products/auto-rig-pro But be prepared to still fix something manually.

And finally: do not let the frustration beat you 



and have fun 



 Animations from Mixamo